Method for adding additives to molten steel



May 30, l967 HAJIME NAKAMURA 3,322,530

METHOD FOR ADDING ADDITIVES TO MOLTEN STEEL Filed Aug. 23, 1963 CSCDE 0FA40/.TEN STEEL nifl/ 1115411 A.

" MrHoDs 0F m T 3 u n m M n M m N l S WJ l D m vN m l m w w m 0 Mw L U BH L ST. A 6 ME. R M2M w x/ M o 4K w .a m m 75 1.0 @Aw/UM cYANAM/DE ADDED(ir/row oF MoLTE/v STEEL) This invention relates to a method for addingadditives to molten steel. More particularly, this inven-tion relates toa method for adding additives to molten steel refined by any way ofoxidation refining or both oxidation and reduction refining, the steelbeing tapped to a ladle equipped with a suitable number of transportpipes in which the additives are provided or through which they are fedto said molten steel. Said transport pipes are so designed as to beconsumed in the molten steel from its lower end in approximatesynchronism with the rise of said molten steel surface as the latter isaccumulated in the ladle in such a manner as to release said additivesgradually and steadily in a zone of 4the molten steel bath that isapproximately constant in relative position with respect to the risingsurface of said steel bath. The primary purpose of this invention is toprovide low carbon unalloyed steel, or low carbon low alloyed steelenriched with nitrogen, and also to provide a process to produce suchsteels economically and efiiciently as well as to provide a newsteelmaking process in which the reduction refining may be carried outduring and simultaneous with the tapping. Other purposes andapplications of this invention will be elucidated in the course ofdisclosure.

As for effective and economical industrial methods for increasing7 thenitrogen content of molten steel, I have previously proposed four ways:(l) a method in which the nitrogen gas or nitrogen containing inert gasis blown directly into the steel bath under a pressure that isappropriate to give rise to a proper stirring motion within the bath,(2) a method in which a nitrogen compound is blown into the bath onstream of nitrogen or inert gas, or a gaseous mixture thereof, (3) ame-thod in which calcium cyanamide which is specifically doped with adeoxidation agent is blown into the bath on stream of inert gas ornon-oxidizing gas (these three methods will be referred to as the bathmethod hereinafter for the sake of simplicity), and (4) a method inwhich nitrogen compound is fed on stream of nitrogen gas or inert gas tothe bottom portion of the slag layer immediately above the surface ofthe molten steel bath, namely to the interface between the slag and themelt (this method will be referred to as the interface method).

These methods are all immediately applicable to a large industrialsteelmaking furnace such as an open hearth furnace, large electric arcfurnace or pure oxygen overblowing (the LD) converter furnace, and byusing any one of these methods the molten steel in a quantity of as muchas l() tons or over can be enriched with nitrogen to a desired nitrogencontent within a few minutes, or at least minutes. However, it is not tobe disregarded that even such a short nitrogen enriching stage canbecome a substantial burden, when the very shortness of the entiresteelmaking time is considered to be of utmost importance in such caseas, for example, the pure oxygen overblowing converter process (the LDprocess).

tes Patent The difficulty can be overcome by carrying out the nitrogenenrichment at the ladle, in other words, in the molten steel which isbeing tapped or already tapped and in the ladle. For this practice, anumber of methods has been proposed. For example, s-teel smel-ted byhigh frequency furnace up to l tone capacity and tapped into a ladle waseffectively enriched with nitrogen up to about 0.038% by adding nitrogencontaining ferromanganese or water-free calcium nitrate (H. J. Wiesteret alii, Stahl und Eisen, volume 77, pages 773-784, 1957). (This methodwill be referred to as the throw-in method hereinafter.)

However, since steels tapped into a ladle by conventional industrialmethod are usually often covered by a thick layer of slag, large amountsIof nitrogen enrichment agent thrown into the steel are oxidized andlost upon passing through the slag layer. Therefore only a low andunstable nitrogen yield can be gained.

Alternately, the nitrogen enrichment agent may be previously laid alongthe bottom of the ladle and the molten steel may be tapped onto saidagent in the same way as commonly employed for adding various alloyingelements. (This method will be referred to as the static method.) Inthis case, although the efficiency and the stability of nitrogenenrichment may be improved over the throw-in method, the reaction of thenitrogei/fenrichment with molten steel tends to take place violently,and dangerously, so tha-t a portion of the agent inevitably flows awayand is lost. Therefore, the efficiency and the precision of nitrogenenrichment is far less than those of this invention, to say nothingabout the incapability of the static method to attain a nitrogen contentover a certain limit.

The above described bath methods and the interface method are applicableto steel being tapped or. already tapped into a ladle. However, theseprocesses are not devoid of shortcomings. Namely, as -the molten steelbath in a ladle is often over a few meters deep and hence the uniformityof nitrogen content throughout the steel may not always be assured and,further, the manipulation of the conduit pipe for the nitrogenenrichment agent is not easy as it is at the front of a steelmakingfurnace.

In order to overcome the difficulties described above, I have discoveredafter many tests and experiments that when a pipe, so designed as topossess a consumption (melting) rate which is approximately the same asthe rate of the rise of the metal surface within the ladle, and t0maintain its free outlet end at la certain depth relative to the risingmetal surface, is used either as the conduit pipe through which thedesired additives are to be fed into the melt or as a container in whichsaid material is previously stored (such a pipe will be simply referredto as the consumable transport pipe), one can perform the addition ofadditives or the nitrogen enrichment operation quite easily,economically, and with remarkable efficiency without affecting thesteelmaking time itself in any way.

As for the nitrogen enrichment agent to be used in this method,agricultural calcium cyanamide is most satisfactory,v but other nitrogencontaining compounds that readily dissociate at a high temperature toliberate nitrogen gas, such as manganese nitride, chromium nitride,lithium nitride, or nitrogen containing ferroalloys of these elements,can be used just as well. Additives include elements that are commonlyreferred to as the norm-al alloying elements, such as nickel, chromium,molybdenum,

vanadium, copper, `and others, or metallic elements that are to serve asthe nitrogen fixer, such as aluminum, beryllium, or columbium, ordeoxidational alloying elements such as aluminum, ferrosilicon orferromanganese, or deoxidizers such as calcium carbide. Those additivesmay conveniently be in a readily meltable form such as powder, granular,ribbon or tablet.

These additives or nitrogen enrichment agents (these will be simplyreferred to as the additives hereinafter) may be previously stored inthe consumable transport pipe (the consumable container method), or theymay be fed into the molten steel through the consumable transport pipeon stream of a suitable gas such as nitrogen, inert gas ornon-'oxidizing gas (the gas-blow method).

One factor rather critical in practicing this invention, which was foundin the course of trials, is to hold the relative depth of the freeoutlet of the consumable transport pipe at about 5 cm. to 75 cm. fromthe rising surface of the accumulating molten steel. This can beachieved by, for example, selecting the wall thickness and diameter ofthe pipe so that natural rate of its melting in the steel bath from thelower end is approximately the same as that of the rise of the moltensteel surface. The required wall thickness and the diameter can be foundby calculations, but one can readily determine a proper pipe -after afew experiments or trials. When the relative depth is too small, thecarrier gas tends to splash the additives out into the air, or else, thenitrogen liberated from the agent tends to escape before being absorbedin the steel suciently, and a main portion of the nitrogen is lost.Because of the violent lagitation at the melt surface and the overlyingslag layer, an exact measurement is impossible. Nevertheless t Was foundempirically that the presence of a molten steel layer of at least 5 cm.above the free end of the consumable pipe assures satisfactory nitrogenenrichment.

I shall now describe operational examples in which the principle of myinvention was applied to the electric arc furnace steelmaking process asillustrated in the accompanying drawing in which:

FIGURE 1 is a schematic drawing of a ladle in vertical cross-section forpracticing one embodiment of this invention;

FIGURE 2 is la diagram to show the efficiency of nitrogen enrichment ofmolten steel due to this invention as compared with that due to a knownmethod;

FIGURE 3 is a schematic drawing of a consumable transport pipe invertical cross section that is convenient for slag reduction rening ofthe molten steel at the ladle; and

FIGURE 4 is a schematic drawing of -a ladle in vertical cross-sectionfor another embodiment of this invention.

FIGURE 1 illustrates schematically an exemplary large ladle equippedwith a steel pipe for a convenient practicing of the gas-blow method ofthis invention, wherein 1 is the accumulating molten steel bath, 2 isthe heat resistive lining of the ladle, 3 is the stopper, 4 is thehanger handle of the ladle, 5 is the consumable transport pipe whoseconsumed portion is indicated by broken line. Although in the gure, theconsumable transport pipe 5 is fixed against the hanger handle of theladle 4, this pipe may equally be well held by any other means, forexample, by human hands. FIGURE 2 shows the nitrogen yield in theproduct steel as function of the amount of nitrogen enrichment agent(calcium cyanamide) added per ton of molten steel. The solid lineindicates the results due to my methods while the broken line representsthose due to a known method.

Example 1 As soon as the tapping of steel into the ladle was commenced,an agricultural calcium cyanamide of thexcomposition listed in Table 1lwas blown into the melt on nitrogen gas under a pressure of about 1 to1.5 kg./cm.2 according to the gas-blow method. As the tapping period wasslightly over l minute for a 3 ton heat and slightly over 2 minutes forla l0 ton heat, the feeding rate of the calcium cyanamide was soadjusted that the desired quantity would be fed to the melt in slightlyless than 1 minute or slightly less than 2 minutes in the respectivecases.

The dimensions of the ladle were about 100 cm. in diameter at the upperopening and about 120 cm. deep for 3 ton heat about 115 cm. in diameterat the upper opening and about 160 cm. deep for 10 ton heat. Thediameter and the wall thickness of the consumable transport pipes usedand other pertinent operational data are presented in the Table 2 underCharges 1, 2, 5 and 6. Actual measurements showed that the relativedepth of the outlet end of the consumable transport pipes was in any oneof the cases always about 20 cm. to 30 cm.

Immediately succeeding the completion of tapping, metallic aluminum forCharges 2 and 5, columbium for Charge l, and beryllium for Charge 6 wasadded to the melt. This was done in order to convert the nitrogen intobenecial metallic nitride which is to precipitate out throughout thegranular structure of the steel during the hot working process or heattreatment process that was to follow, thereby making the steel highlyductile. The results thu-s obtained are summarized in the Table 2 as theCharges 1, 2, 5 and 6, and in FIGURE 2 by single circles.

TABLE 1.-CHEMICAL ANALYSIS OF AGRICULTURAL CALCIUM CYANAMIDE, PERCENTCaCN2 55 cao 33 C 12 TABLE 2.-CHEMICAL ANALYSIS OF STEELS AND OTHEROPERATIONAL DATA Nitrogen Content, Percent X10 3 Nitrogen Yield, CarbonContent, Percent X10 2 Molten Calcium Percent Charge Steel, Cyanamideton Added,

kg./ton Before Increment Apparent Net Before In Increment ProductProduct asaaeo TABLE Z-CHEMICAL ANALYSIS OF STEELS AND OTHER OPERATIONALDATA-Continued Soluble Aluminum Content, Percent Other Element, PercentDimensions of Consumable Transport Pipes, mm.

Charge Added ln Product Yield Si Mn Others Outer Wall Diameter Thicknessl The gas-blow" method of this invention. 2 The "consumable containermethod of this lnvention. 3 The known static method.

Example Z Electric arc furnace molten steels of 3 ton and 10 ton weretreated by the consumable container method. The same calcium cyanamideas the foregoing example was used for the nitrogen enrichment agent.Granular aluminum (Charge 3) or aluminum, nickel, chromium, molybdenum,vanadium (Charge 4) or vanadium (Charge 7) were thoroughly admixed withthe calcium cyanamide. The mixture was then compactly stuffed into theconsumable transport pipe of diameter and wall thickness as listed inthe Table 2 under Charges 3, 4 and 7 until the apparent specific gravityof the calcium cyanamide became about 1.2 to 2. The holding position ofthe pipe was the same as in FIGURE 1. Although both ends of the pipewere sealed in this example, it was found that the ends may be leftopen. Other factors such as the tapping time, the relative depth of theoutlet and such were the same as in the Example 1. The results from thisexample are summarized in Table 2 as Charges 3, 4 and 7, and in FIG- URE2 by the double circles.

Another factor that may become critical in practicing my methods isthat, depending on the state of on-rushing molten steel cascade, specialcare may have to be taken to protect the transport pipe from untimelymelt down due to molten steel splashes. This can be done in many ways.For example, the pipe may be made in double structure or it may beencircled by a suitable material which is heat resistive enough atrelatively lower temperatures but readily melts away at the temperatureof the molten steel.

In order to outline the merits of my methods over those of known methodsfor nitrogen enriching the molten steel, the static method which isoften practiced today for its apparent simplicity and fairly highefficiency was applied to the electric arc steelmaking process. Namely,the nitrogen enrichment agent, the same calcium cyanamide of theprevious examples (Table l), was laid on the ladle bottom onto which themolten steel was tapped.

The results are summarized in Table 2 as Charges 9 through 12 andpresented in FIGURE 2 by symbol crosses. The particulars of the smeltingfurnace, the steelmaking process, the dimensions of ladles, the tappingtime and other pertinent factors were the same as in the previousexamples. Metallic aluminum was added into the melt immediatelyfollowing the completion of tapping.

Examination of Table 2 and FIGURE 2 clearly shows that (l) the nitrogenenrichment of molten steel may be executed by either one of my methodsor the known static method; however, (2) that my two methods and theknown method differ greatly in the efficiency of nitrogen enrichment,namely, while the nitrogen yield in either one of my methods is 32.6%,in the static method it is only 21.8% (Table 2); (3) that the nitrogenenrichment efficiency of my methods is very stable or constant up to thesaturation nitrogen content of about 0.035% to 0.040%, Whereas it tendsto tail olf for nitrogen contents of over about 0.015% in the staticmethod, this being probably because a substantial portion of thenitrogen enrichment agent heaped up on the ladle bottom gets burnt upbefore ever having an opportunity to react with the molten steel; (4)the yield of aluminum is 90.7% in the consumable container method and68.6% in the gas-blow method of my invention, whereas it is only 53.3%in the static method (Table 2); (5) although carburization of steel dueto the carbon content of nitrogen enrichment agent is unavoidable,particularly when the agricultural calcium cyanamide is used as such,the extent of the carburization due to agricultural calcium cyanamide inmy methods are well controlled as to be perfectly predictable, whereasin the static method it is so random as to make exact predictionimpossible (Table 2).

The present methods of nitrogen enrichment are superior to my otherprevious methods, the bath method and the interface method which arementioned earlier, if the intrinsic difference in their respectiveapplicability is disregarded. Namely, (l) since the nitrogen enrichmentof the present methods is performed on the melt being tapped, entirelyindependent of the smelting process itself, the entire steelmakingduration suffers no prolongation whatsoever despite the #addition ofthis step; (2) the hardship that must be imposed upon the operator inhandling the conduit pipe in front of hot furnace in the bath orinterface method is entirely avoided in the present methods; (3) wherethe nitrogen yield on the agricultural calcium cyanamide is about 17.5%(oxidation slag) to about 21% (reduction slag) in the bath method orabout 15% in the interface method, it is as high as about 32.6% in thepresent methods; (4) where the aluminum yield is about 25% when forciblyadded into the melt at the ladle after having been enriched withnitrogen by either the bath or the interface method, it is as much asabout 68.6% (gasblow method) or about 90.7% (consumable containermethod); this being probably due to the fact that the steel, tappedwhile being subjected to the steady interaction with the calciumcyanamide, is in a state of more thorough deoxidation by the reactionwith the dissociation product from the the ladle', thereby improvingoxidizers greahy'while lreducing the entire ysteelrnakingA time by alarge margin. f f

' Aiinng at "the furnace,

three consumable containers which are schematically ily lust'rated inFIGUREl 3. In FIGURE of linner ydiameter about 13 om.

'ness and diameter calcium cyanamide, a factor .that will not fail toassurey y'a' high yieldfor elements which otherwise react with oxygen insteel and are lost as nonmetallic,y inclusion.-

'Inasmuch as the principle of this` invention can be directly applied toother operationm'for example, addif tion of deoxidizer, an entirely new.method of steeimalcing.

j limmediate reactionwithl the latter, while inthe conven- Words, .the.deoxidizelg Athe calcium carbide in this case, reacts with the moltensteel without ever touching air and- Ational methods, the deoxidizerinevitably vcomes 'in contact with air beforel it begins reactingwith.steel. In other p reduces the steel thoroughly' andilocally. in the siagreducis here evolved. Namely, I have succeeded in performing thereduction.

of this inventionl to the steel which is being tapped-to Example, 3y

A110 ton charge composedy mainly of scrap steel was melted in the sameelectric arcfurnace. as lin the .forel going examples andsrneltin theknown method .of oxida-` reiining process by. applying the methods theyield of various ie-` tion refining by using pure oxygen gas; Thehcatwas then tapped, without performing the. customary lreductionreintoaladle Athat was equipped with 4.5 cm.' whose `both ends .areclosed, l'7 andlil are tbe `layers ofdeoxidizercalcium. carbide of aboutl0 cm.thick Acompactly stuffed to anapparent specific gravity of aboutv2.0, .Sand 11 are the layers' ofgranularv `'are to act partially as adeoxidizer andpartially as an alloying element, 9 and 12' areA thelayers .of granular fer.v

alloying element,- arrd 13 the .same 'manuel'. ias describedl .in ltheExample y2; (the consumable contair'ie'r.l method). The .relative depthyof. the container outlet was always about to 30 cm., whie the otherfactors such as the dimensions of the ladle, the manner of containerpipe holding, the time of tapping were all the same as those in theprevious examples.

Immediately following the completion of tapping, that is the completionof reduction refining in this particular case, the heat was teemed intoingot in an ordinary way.

TABLE 3.-CHEI\IICAL ANALYSIS OF STEEL, PERCENT Table 3 which presentsthe chemical composition of the product steel in comparison with thatbefore tapping indubitably indicates that an effective reductionrefining was achieved. The merit of this method may be seen in thefollowing points in respect to a normal electric arc furnace smeltingprocess. Namely, (l) the time needed for the reduction refining processin this method is nothing more or less than the time needed for tapping,namely only about 2 minutes at most, whereas it is at least a few tensof minutes when performed at the furnace according to the normal way ofsmelting, (2) the yield of various additives is about 85% for silicon,about 92% for manganese, about 89% for aluminum in this method, whereasit is only about 63% for silicon, about 60% for manganese and about foraluminum in the customary method, (3) therefore, the ease and theaccuracy of controlling the additives, and the economy of fuel orelectric power associated with this method are evident.

The primary factor that makes this method of reduction refining verysuccessful appears to reside in the gradual and steady delivery ofdeoxidizer into the melt and its `3, .6 is av steel `pipe and wallthickness about -ji aluminum thaty l thick-l 'conveniently be employedinpa'ralle'l orseries disposition,l

- Althoughatransport pipe withl both port: pipe-for -calciumearbide gport. pipe; for other additives' which Awould lbe `ferrosilic'on,

f ferromang'anese andi aluminum had this variety been ap,-A j pliedl tothe foregoing'example. The Wall .thickness land '35,

` tion yreaction i into kwhich `portion f thek other additives arer fnow. delivered to enjoy a remarkablyimprovedyieldThis f lprocess isyevenly diffusedl throughout the steel bath kby the stirring movementofthe bath itself. f

ends. closedA containing ay 'stratification' of Avarious additives.composed of lwell defined layers was used in the above example and 'inFIGURE 3, the ends of the pipe need not be closed or the additives needynot be clearlystratied. yIn fact,either one or both the ends -canbe leftopen' andfthcadditives i may =be stuifed so ast-'o find themselvesalong-fafpredeterv mined concentration gradient. Alternatively,thenumber` Aandthe 'relative positionof the` consumable transportpipes.f

,may be kso chosen `that during la predetermined. period in the 'tappingonly the icalcium carbide is ldelivered to `performthe deoXidatio-nrainning, then. upon. closure of l -V .that stage,` or overlappingiwith'thelatterperiod thereof,

the other transport pipes-for remaining; additives startk vparticipating; For this .variety-, a number of .pipes may in aym'annerthat is shown schematically inl FIGURE 4 in y way' of:illustration.l In. FIGURE. 4;, 14 is theladle, l15 isy thelevel themolten steel: is expectedto attain ultimatelyl 16 is the consumabletrans-y l17 is the consumable trans-y on. the; completion of tapping,

the :diameter of thesepipes maybe selectedindividually accordng'tofthei'r'pa'rticular requirements, yand they need not be the same.'

, y The improvement 'in the cleanness of the ingot or the l ddiminira'tionl of the ingotisms-gained. in my other methods lof the bathor the interface method, that is considered attributable to the actionof carrier gas, is also obtained in this new method due probably to theaction of the liberated nitrogen and the stirring motion of the moltensteel itself in conjunction with the proper usage of gas and additives.

Having disclosed heretofore the principle of my invention and severalexamples in which the principle was 'applied in practice, it will beimmediately and indubitably understood by all those who are skilled inthe art of smelting of steel or indeed of any metal that the principleand applications of my invention are not to be limited to those specificexamples given and that many modifications are possible withoutdeviating from the principle of my invention, which is only to bebounded by the appended claims.

I claim:

1. A method comprising adding a nitrogen enrichment agent to moltensteel which is being tapped into a receptacle and which thereby has acontinuously rising surface by supplying the agent to the molten steelby a consumable pipe which is placed in said steel and is consumed bythe steel at a rate to supply said agent thereto at a lower dischargeend of the pipe which continuously rises and remains at a depth of 5-75cm. relative to said rising surface of the molten steel.

2. A method as claimed in claim 1, wherein the nitrogen enrichment agentis injected into the steel via said pipe by means of an inert gas.

3. A method as claimed in claim 2, wherein the gas is blown into themolten steel at a pressure of about 1.0 to 1.5 kg./cm.2.

4. A method as claimed in claim 1, wherein the nitrogen enrichment agentis loaded into the consumable pipe.

5. A method as claimed in claim 1, wherein the pipe is charged withlayers of deoxidizer and alloying metals.

6. A method as claimed in claim 5, wherein the pipe is a steel pipe withan internal diameter of about 13 cm. and a wall thickness of about 4.5cm.

7. A method as claimed in claim 1, wherein the enrichment agent iscalcium cyanamide.

8. A method as claimed in claim 7, wherein the calcium cyanamide isadded in an amount of about 0.7-3.5 kg./1 ton.

9. A method as claimed in claim 1, wherein the tapping time is about 1-2minutes.

10. A method as claimed in claim 1, wherein at least one end of the pipeis sealed.

11. A method as claimed in claim 1, wherein the pipe is protectedagainst premature consumption due to splashing steel with a materialwhich is heat resistive at relatively loW temperature but which isconsumed in the molten steel.

12. A method as claimed in claim 1 comprising enriching the steel up toabout 0.040% nitrogen content.

13. A method as claimed in claim 1, wherein said steel pipe has an outerdiameter of about 20-130 mm. and a wall thickness of about 2.8-45 mm.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 785,551 10/1957Great Britain. 874,181 8/1961 Great Britain.

20 HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner. H. W. TARRING, Assistant Examiner.

1. A METHOD COMPRISING ADDING A NITROGEN ENRICHMENT AGENT TO MOLTENSTEEL WHICH IS BEING TAPPED INTO A RECEPTACLE AND WHICH THEREBY HAS ACONTINUOUSLY RISING SURFACE BY SUPPLYING THE AGENT TO THE MOLTEN STEELBY A CONSUMABLE PIPE WHICH IS PLACED IN SAID STEEL AND IS CONSUMED BYTHE STEEL AT A RATE TO SUPPLY SAID AGENT THERETO AT A LOWER